1. Field of the Invention
The present invention relates to computer-based tools for manipulating digital images. More specifically, the present invention relates to a method and an apparatus for deposterizing a posterized digital image.
2. Related Art
Unlike conventional film-based images, digital images can be loaded onto a computer system. This allows the digital images to be edited and otherwise manipulated using computer-based image-editing tools.
These image-editing tools are often used to manipulate posterized images, which have a reduced number of discrete levels in each color channel. Hence, these posterized images have fewer distinct colors (red, green, and blue) as compared with non-posterized images, which can have color values that span the full range of possible color values (from 0 to 255). For example, a posterized image can include: a black-and-white 1-bit image; a 16-bit image that originates from a 12- to 14-bit device; or an 8-bit image which has been converted into a 16-bit image.
Posterized images can be problematic. One problem is that they lack pixel depth and certain areas are void of detail. Existing image-processing techniques can be used to apply adjustments to posterized images to bring out the expected detail, but unfortunately these existing techniques produce only limited results. Furthermore, applying other image processing techniques to posterized images often produces artifacts due to jumps in pixel values. For example, “image-sharpening” tools (such as the “Unsharp Mask” in Adobe Photoshop™) are adversely influenced by the posterization “steps” in the image. These image-sharpening tools can produce unnatural results because they outline the “steps” instead of sharpening the image.
To avoid the above-described problems, people try to “blur” posterized images. For example, a Gaussian blur of radius 1-2 pixels can improve the histogram of color values for the image somewhat, so that now the image is essentially not posterized, even if some traces of posterization remain. However, we pay a price in blurring an image because the blurring process causes image details to be lost. Complete removal of posterization currently requires more than a 2-pixel Gaussian blur, which is excessive and bad for the image. Consequently, at present, people have to live with the posterization-versus-blur tradeoff.
Furthermore, because information is lost during the blurring process, it is not possible to restore the original posterized image from the “blurred” image. Hence, what is needed is a method and an apparatus for deposterizing an image without the above-described problems.